A student has carried out a reaction to produce an organic liquid product. The crude product contains unreacted starting materials, water, and traces of acid catalyst. Describe how the student could purify and dry the organic liquid to obtain a pure, dry sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In organic synthesis, the crude product mixture must be processed after the reaction to remove impurities before a pure sample can be collected.
Model answer (5 marks)
1. Use a separating funnel to wash the crude product with water, removing water‑soluble impurities such as unreacted starting material and acid catalyst.
2. Separate the aqueous layer from the organic layer by opening the tap of the separating funnel.
3. Add a drying agent (e.g. anhydrous calcium chloride, magnesium sulfate or sodium sulfate) to the organic layer to remove any remaining water.
4. Filter or decant the dried organic liquid to remove the solid drying agent.
5. Distil the organic liquid and collect the fraction at the expected boiling point to obtain a pure, dry product.
2. Separate the aqueous layer from the organic layer by opening the tap of the separating funnel.
3. Add a drying agent (e.g. anhydrous calcium chloride, magnesium sulfate or sodium sulfate) to the organic layer to remove any remaining water.
4. Filter or decant the dried organic liquid to remove the solid drying agent.
5. Distil the organic liquid and collect the fraction at the expected boiling point to obtain a pure, dry product.
Examiner tips
- Use the correct order of operations – wash, separate, dry, filter, distil. Mention the purpose of each step (removal of water‑soluble impurities, removal of water, collection of pure fraction).
- Include the names of common drying agents and the expected boiling point of the product.
Mark scheme (5 marks)
- Use a separating funnel to wash the crude product / shake the crude product with water in a separating funnel to remove water-soluble impurities (unreacted starting materials and acid catalyst)
- Run off / separate the aqueous layer from the organic layer using the separating funnel tap
- Add a drying agent (e.g. anhydrous calcium chloride / anhydrous magnesium sulfate / anhydrous sodium sulfate) to the organic layer to remove traces of water
- Filter / decant the dried organic liquid to remove the solid drying agent
- Distil the organic liquid and collect the fraction at the correct / expected boiling point to obtain the pure product
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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